Thermal or heat activated laminating films are constructed of a layer of Biaxially oriented polypropylene base film and a layer of adhesive resin (Ethylene Vinyl Acetate (EVA) or Polyethylene or Ethylene Acrylic Acid (EAA) or a blend of all these), bonded together. During the lamination process, the adhesive resin is softened or liquefied and permanently bonded to the document. Once liquefied, the adhesive is spread across the surface and with pressure, forced into the pores of the paper. The adhesive then hardens as it cools creating a permanent bond between paper and film.
Pressure Sensitive laminating films are often referred to as cold films. The reason is that they require no heat or very limited amount of heat to adhere to any particular document during the laminating or mounting process. The adhesive layer in cold film is constantly in a tacky state since no heat is used in the sealing process (only pressure). This gives the need for a paper or film liner, that keeps the film from adhering to itself when it is rolled. Pressure sensitive laminating films will adhere to almost any substrate but is best used for heat sensitive or previously heat activated materials. These films work great for overlaminating and mounting on such substrates as gator board, foam board, wood, Plexiglass or delicate print materials such as photostats, transparencies, cyberchromes and thermal transfers.
Extrusion coating is a variation of the extrusion process. In FIG. 2, a substrate comes in on the left, a primer is applied and cured, and another material is applied over that from the extruder.
Extrusion coating followed by lamination is used to protect printing. It puts a layer of material over the printed surface to prevent any scuffing or smearing. It can also be used to bond materials that can't be co-extruded, materials with very different melting points, or materials that can't be extruded like paper, foil, metalized films or oriented films.
Lamination is a process of pasting two different or similar substrates together to achieve hybrid properties. In case of book cover lamination, printed paper and plastic film are laminated to protect the paper against weather, water, stain and tear. This lamination is generally done by coating adhesive over plastic film and then laminating it with printed paper or paperboard. This process, which is called as “Wet Lamination” is widely used world over and predominantly in India. In USA and in few European countries, the process employed is different. Plastic films such as BOPP, Polyester or Nylon are extrusion coated with low density polyethylene (LDPE), ethylene vinyl acetate (EVA) or EAA or a blend of all these, etc. This coated film and printed paper are passed over heated rolls under pressure and thereby inducing lamination. This process is called “Thermolamination”. Industry refers this as dry lamination.
Thermal laminating materials and methods are known for protecting printed substrates by adhering a protective thermoplastic polymer cover film or sheet to one or both of the major surfaces of a printed substrate. Thermal film lamination provides immediate benefits to printed materials-most notably, an attractive look and polished or a lacquered feel. With thermal lamination, printed materials enjoy greater durability. Unlike UV or other coatings, thermal lamination provides protection against scratching and smudging. The life of the printed materials is extended in a number of ways because thermal lamination make paper more tear resistant and protects inks on printed substrates.
Based on various applications, the following features are required in the film.
Film for thermal lamination should be made in both glossy & matte variance. Thicknesses for glossy should be 22-80 microns. The same should be for matte variance also.
Laminating surface should be low melting, polar & easy flowing. The opposite surface should be hot slip modified without any locking issue when laminated on both surfaces, good scuff resistance, good antistatic property to enable better & smooth runnability at elevated temperatures.
Unlike other finishes, thermal lamination is a very safe and waste free process. It does not emit harmful volatiles and other substances into the atmosphere. Accordingly, thermal lamination is a significant breakthrough in finishing practices and contributes greatly toward preserving the environment. Earlier, less productive solvent- and water-based systems are now being changed over to thermal lamination.
The process of thermal lamination uses heat, pressure and thermoplastic material to combine substrates without an in-line adhesive application. Webs of paper, paperboard, etc. are bonded with BOPP film using polyolefin thermoplastics as the bonding medium. Sufficient heat and pressure are momentarily introduced to nearly melt the thermoplastic surface at the laminating nip and create a mechanical bond. Such a coating material should also be able to make a very good bond with printed substrate, also. Thus, bonding is as much chemical as it is mechanical.
The laminating section of the roll fed thermal laminator consists of large diameter heated rolls, nip rolls, preheat rolls, idlers, spreader roll and chill rolls. The heated steel rolls are driven, and have a double-wall design, and are Teflon coated. This allows for a controlled, balanced flow of large volumes of heating media for uniform temperature and dwell time on the heated rolls. Electrical heating system is also available which is controlled by thermocouple inserted in to the rolls.
Films used in thermal lamination are run over web-driven, fixed-bow rolls, which keep them wrinkle free and flat as they enter the lamination nip. After the lamination is complete, the web is transported over two or three driven, double wall, flow-through, chrome-plated-steel chill rolls. The web is cooled to room temperature to solidify the thermoplastic layer and complete the bonding process.